A more thorough explanation:

Diplomatic protection is a term used to describe the protection given by a country's representatives to one of its citizens against another country's alleged violation of international law. This protection can be provided by diplomatic agents, missions, consuls, and even military forces.

It is important to note that diplomatic protection is not limited to just protecting citizens. Diplomatic or consular actions to obtain concessions or other government contracts for nationals from the receiving State, or the arrangement of legal defense for a justly imprisoned national are not diplomatic protection in our sense; they are usually neither directed against the other State nor based on a real or alleged violation of international law.

For example, if a citizen of Country A is arrested and detained in Country B without a fair trial, Country A may provide diplomatic protection to its citizen by intervening on their behalf and requesting that Country B release the citizen or provide a fair trial. Another example could be if a citizen of Country A has their property seized by Country B without just compensation, Country A may provide diplomatic protection by requesting that Country B return the property or provide just compensation.